1. Field of the Invention
This invention has as its object a protective circuit for a control circuit. It finds a preferred application in the control of liquid crystal display screens.
2. Discussion of the Background
A liquid crystal display screen is generally in the form illustrated in FIG. 1. The screen itself, ECR, consists of addressing lines L and columns C, of a matrix of pixels P, each connected to a transistor TFT whose state is controlled by an associated line L and column C.
Such a screen is controlled by a line control circuit CCL, which sequentially applies an addressing voltage (for example, several volts) to the lines, and by a column control circuit CCC, which applies, to all the columns, voltages reflecting the light intensity of the points to be displayed on the addressed line. The overall image is thus displayed line by line.
Column control circuit CCC receives a video signal SV delivered by a video circuit CV. This signal generally consists of three components corresponding to the three primary components of a color image.
If screen ECR has 162 columns, circuit CCC comprises 162 elementary column control circuits, placed in parallel, and 162 outputs connected to various columns. Each elementary column control circuit (also called "driver column" in the technical literature) comprises a sample and hold circuit whose function is to sample the video signal at a given moment corresponding to the column to be controlled and to hold this sample on the column for the entire addressing period of a line ("sample and hold" function in English terminology).
This invention relates to such a sample and hold circuit.
Most liquid crystal display screens require a control voltage on the order of 12 V peak to peak. The recourse to a control circuit supporting such a voltage would limit the possibilities of integration as well as the operating speed. Now, for large-size screens, a high integration density is necessary and a high control speed is desirable.
It is therefore preferred to work with circuits delivering only 6 V peak to peak, but by providing devices on the screen, such as the use of a counterelectrode. By reversing polarity applied to the counterelectrode, it is possible to use a voltage excursion going respectively from 0 to 6 V and from -6 V to 0, in other words to using 12 V peak to peak (from -6 V to +6 V).
However, these devices present the drawback of bringing parasitic voltages to the control circuit, mainly at the time of switchings. An output transistor can thus see 12 V between its drain and its source. Breakdown and avalanche phenomena appear, which result in deterioration of the circuit.